Modern high frequency (HF) and other communication systems require extremely linear broadband amplifiers to provide secure jam-free transmissions. Non-linear amplifiers are characterized by the generation of harmonic distortion of the fundamental signals driving the amplifiers and by the generation of intermodulation products between two or more fundamental signals. Obtaining low levels of harmonic and intermodulation distortion is much more difficult in broadband amplifiers than in amplifiers with tuned selectivity.
The practical realization of linear broadband amplifiers is possible only by employing feedforward cancellation. Feedforward cancellation techniques include passive cancellation which provides an improvement of 20 to 30 dB in linearity over the HF band of 2 to 30 MHz and adaptive cancellation which provides deep null cancellation in the cancellation circuits of residues of the fundamental signals being transmitted. The deep null cancellation can be maintained over the dynamic transmission bandwidth of the fundamental signal. Thus, deep null cancellation in the cancellation circuits is important because it improves the overall cancellation obtainable for the amplifier and it permits a reduction in the size of the cancellation amplifiers, thereby improving the general efficiency of the amplifier.